The use of powder metallurgical techniques in the production of metal parts is well established. Such techniques typically involve mixing iron-based powders with an allowing element such as graphite, copper, or nickel in powder form, compacting the mixture in a die, ejecting the compact from the die, and sintering of the compact. The presence of the alloying element permits the attainment of strength and other mechanical properties in the sintered part which could not be reached with iron-based powders alone.
The alloying ingredients which normally are used in iron-based powder mixtures typically differ from the basic iron-based in particle size, shape, and density. For example, the average particle size of the iron-based powders used in the manufacture of sintered metal parts is typically about 25-150 microns. In contrast, the average particle size of most alloying ingredients used in conjunction with the iron-based powders is less than about 75 microns and often less than about 20 microns. Alloying powders are used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solid state diffusion during the sintering operation. However, this extremely fine size, together with the overall differences between the iron-based and alloying powders in particle size, shape, and density, make these powder mixtures susceptible to the undesirable separatory phenomena of segregation and dusting. Binding agents frequently are added to bond the powder particles and reduce the segregation. For example, U.S. Pat. No. 4,834,800, in the name of Semel, discloses certain water-insoluble resins as binding agents.
Lubricants can also be admixed with a powder blend to reduce internal friction between particles during compaction, to permit easier ejection of the compact from the die cavity, to reduce die wear, and/or to allow more uniform compaction of the blend. Common lubricants include solids such as metallic stearates or synthetic waxes. U.S. Pat. No. 4,106,932, in the name of Blanchford, discloses the use of certain liquid lubricants in microencapsulated form.
As will be recognized, most known lubricants reduce the green strength of the compact. It is believed that during compaction the lubricant is exuded between iron and/or alloying particles such that it fills the pore volume between the particles and interferes with particle/particle bonding. Indeed, some shapes cannot be pressed using known lubricants. Tall, thin-walled bushings, for example, require large amounts of lubricant to overcome die wall friction and reduce the required ejection force. Such levels of lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. To avoid these problems, it is known to spray the die wall with lubricant rather than to incorporate the lubricant into the powder composition. However, spraying the lubricant increases the compaction cycle time and leads to less uniform compaction. Also, lubricants such as zinc stearate often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive lubricant can lead to compacts having poor dimensional integrity, and volatized lubricant can form soot on the heating elements of a furnace.
Accordingly, there exists a need in the art for metallurgical powder compositions that are resistant to dusting and segregation and that can be readily compacted to strong green parts that are easily ejected from die cavities.